Questions on the symbolic (DSolve, DifferentialRoot) and numerical (NDSolve) solutions of differential equations in Mathematica.

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0
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1answer
53 views

How to properly use conditionals inside NDSolve

Currently I am working with Mathematica to learn the program, and I'm confused what exactly is wrong with this code. ...
1
vote
1answer
76 views

Constants from differential equations

I have a problem, I've solved 2 diff equations and I have: ...
2
votes
3answers
101 views

DSolve with assumptions

I'm new to mathematica and was wondering if someone could help me. I have a differential equation which is: $\frac{dy}{dx} = 1/2 y(1 - y)(a - y + a y)$ It has initial condition $y(0) = y0$ and ...
4
votes
3answers
153 views

How to fit parameters in a system of coupled differential equations

Problem description: I'm studying a chemical reaction that can be modeled by a system of coupled differential equations, and I want to use measured data to determine the parameters that appear in the ...
1
vote
3answers
75 views

NDSolve parametric plot

Here is my code. s = NDSolve[{f'''[x] + f[x]*f''[x] == 0, f[0] == A, f'[0] == 0, f'[10] == 1}, f, x] I want to plot f''[0] ...
5
votes
1answer
167 views

Numerical solution of IVP for linear ODE with variable coefficient runs wild soon

Cross posted in scicomp.SE. A friend of mine showed me this initial value problem (IVP) for a linear ordinary differential equation (ODE) with variable coefficient: $$y''(x)=\left(x^2-1\right) ...
4
votes
0answers
39 views

Strange behaviour of suggestion bar to solve an ODE, potential bug

InputE^(-(1/2) x[t]^2) (Derivative[1][x][t]^2 - (x^\[Prime]\[Prime])[t]) == 0and evaluate, then press the solve ode button on the suggestion bar. It will return ...
3
votes
0answers
64 views

Diverging solution to coupled second order ODEs from NDSolve

For a physics application I am considering the radial Bogoliubov-de Gennes equations in two dimensions. In dimensionless form they read as follows $$ \begin{cases} ...
1
vote
1answer
102 views

How to combine VectorPlot and Plot3D

I have a problem with VectorPlot and Plot3D, and I have a code for both but I dont know how to plot the same figure. Here is the code for Plot3D: ...
0
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0answers
39 views

How to solve a second order non-linear differential equation analytically?

I have the thermoelectric second order non-linear differential equation: Can some one please help me solve this equation. I need to solve for $T$. The two boundary conditions for $T$ are: $\quad ...
2
votes
0answers
86 views

Creating a nonlinear phase portrait

I am putting all of my code in one block for easy copy and paste. ...
0
votes
1answer
45 views

How to obtain coefficients after solving differential equation of order 2 with initial condition?

I have the following answer of a differential equation of order two ...
0
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0answers
34 views

Strange difficulty with DSolveValue

Consider: sys = {u (1 - u + a v), r v (1 - v + b u)} (* {u (1 - u + a v), r (1 + b u - v) v} *) And: ...
1
vote
1answer
46 views

A With and ContourPlot difficulty

Consider: sys = {u (1 - u + a v), r v (1 - v + b u)}; With[{a = 2, b = 3, r = 1}, Evaluate[Thread[sys == {0, 0}]]] Which gives the output: ...
2
votes
0answers
36 views

Axes in a vector plot

Consider: ...
0
votes
1answer
61 views

How I can make the StreamPlot of this differential equation?

I need the StreamPlot of this differential equation but I don't know how. dp/dt=0.4p(1 - p/30), 0 <= t < 5 and 0.4p(1 - p/30) - 0.25p, t > 5
1
vote
2answers
154 views

How can I create function which contains an ODE system and its numeric solution?

I have created a function like the following, which contains an ODE system. And I want to know the effect of different parameters. ...
1
vote
1answer
42 views

Plotting 2nd order ODE solution for specific values of constants

Suppose that I have a 2nd order, linear ODE and I want to plot the solution I found using DSolve. How can I plot giving specific values of ...
0
votes
1answer
38 views

Problems with DSolve [closed]

I'm trying to solve and plot a second order ODE. Here is my code: DSolve[y''[x] + 4 y'[x] + 12 y[x] == 80 Sin[2 x], y[x], x] Problem 1... it sticks me with a ...
0
votes
1answer
59 views

Plot an ODE solution for a list of specific constants [closed]

Let's say I have a 1st order ODE and want to plot the solution. What I have been doing until now is: ...
6
votes
2answers
182 views

Plotting separatrices for nonlinear system

Consider the system: \begin{align*} x'&=(1-x-y)x\\ y'&=(4-7x-3y)y \end{align*} The system has a saddle point at (1/4,3/4). How can I plot the separatrices on the phase portrait having domain ...
5
votes
2answers
147 views

Kernel quits without error in NDSolveValue

I am using the latest 10.1. The following single line command makes the kernel quit without error message (just a beep): ...
19
votes
1answer
508 views

Analogue for Maple's dchange - change of variables in differential expressions

Maple owns an interesting function called dchange which can change the variables of differential equations, but there seems to be no such function in Mathematica. ...
1
vote
1answer
70 views

Error with using NDSolve [duplicate]

I am currently trying to numerically solve a system of differential equations. Here is my code: ...
0
votes
1answer
34 views

Boundary condition not correctly imposed for NDSolve

I notice that when plotting the below NDSolveValue ...
1
vote
0answers
62 views

Failing to solve a simple PDE with NDSolve

I'm trying to solve the following PDE. Actually this is a major simplification of the original with all x,z dependent coefficients being fixed. For some reason Mathematica (10.0) cannot solve even ...
1
vote
1answer
124 views

Solving coupled differential equations with an eigenvalue

I have 2 coupled differential equations with an eigenvalue Ei and want to solve them ...
0
votes
1answer
77 views

Eigen energies of 2D time independent Schrödinger equation

Firstly, I am aware that a similar question of this type was asked, but it was not helpful as it was only one dimensional (so please don't mark my question as a duplicate) The trouble I am having is ...
0
votes
0answers
98 views

Using Runge-Kutta method to find arc length [duplicate]

I don't know much about using the Runge-Kutta method in Mathematica, so I have to ask a straightforward question: How can I use Runge-Kutta to numerically approximate the parameter t of this curve in ...
0
votes
2answers
45 views

Create a random 2x2 matrix with a repeated eigenvalue and single eigenvecor

Does anyone have a good technique for creating a random $2\times 2$ matrix that has one eigenvalue of multiplicity two, but only a single eigenvector? And more generally, has anyone written a ...
0
votes
1answer
121 views

Time evolution of a wave packet from the time-independent Schroedinger equation [duplicate]

Starting off with the time-independent Schroedinger equation (TISE) $\quad \quad -\frac{\hbar^2}{2m} \nabla^2 \psi + V(r, \theta) \psi = E\, \psi,$ I would like to study the time evolution of an ...
0
votes
1answer
51 views

NDSolveValue with Map

I have: ...
0
votes
1answer
91 views

Plotting a NDSolve solution

I'm working on a neutrino oscillations problem, and am running into issues ploting the solution to my differential equations. The equations have non-analytic solutions so I'm using the ...
4
votes
1answer
127 views

NDSolve to solve a PDF

I would like to solve numerically the following PDE: $$ \frac{\partial}{\partial t} p(x,t)=-\frac{\partial}{\partial x}p(x,t)+\frac{1}{2}\frac{\partial^2}{\partial x^2}[x^2\,p(x,t)],\;x\ge0,\;t\ge0, ...
0
votes
0answers
41 views

Integro differential eq boundary difficulties

I'm trying numericaly solve simple integro-differential equation, but have some problems with boundary conditions. System: ...
1
vote
1answer
55 views

Why NDSolve cannot solve such a simple set of differential equations? [closed]

I want to solve a set of ODE like below, but Mathematica outputs nothing for this which greatly confused me. WHY? ...
-3
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1answer
67 views
0
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2answers
72 views

Integrating ODE's

I have: $$ m\frac{dV_y}{dt}=-k\cdot V_y-mg $$ where, $V_y$ is velocity in the $y$-direction. I have the initial condition Vy[0]=40 Sin[40 Degrees] and ...
0
votes
2answers
53 views

Problems solving/plotting a differential equation

I am trying the following: Plot[DSolve[{1220*x''[t] + 1000*x'[t] + 35600 x[t] + 4500*x[t]^3 + 2135 == 0, x[0] == 0, x'[0] == -5}, x, t], {t, 0, 10}] Could ...
6
votes
1answer
99 views

Detecting nearly simultaneous WhenEvents in NDSolve

I am trying to solve a system of (many) coupled nonlinear ODEs. I need to decouple some of the equations (i.e. set the time derivatives of some of the dependent variables to zero) at various points in ...
2
votes
0answers
43 views

Update NumericalFunction in NDSolve

I have NDSolve` StateData from an NDSolve` ...
0
votes
1answer
99 views

How to adjust excel data for NonlinearModelFit?

I'm really new to Mathematica and I'm facing a problem and would really appreciate some help! Maybe my question is similar to one described in How to fit 3 data sets to a model of 4 differential ...
2
votes
4answers
236 views

Solving differential equation

I want to solve the following differential equation DSolve[y''[x] == (λ y[x])/Sqrt[-1 + x], y[x],x] But do not know how to actually solve it. Any suggestion?
0
votes
1answer
77 views

NDSolve::bcedge: Boundary condition not specified on a single edge

NDSolve::bcedge: Boundary condition c[t,5]==Cout is not specified on a single edge of the boundary of the computational domain. >> I'd like to plot $\frac{\partial}{\partial ...
0
votes
0answers
69 views

DSolve response with DSolve

I'm trying this code ...
0
votes
1answer
67 views

Using WhenEvent

I want to solve this simple equation with a event: ...
1
vote
1answer
99 views

Solving ODES in order to find unknown constants

I am trying to solve a set of ODES which represent a second order consecutive chemical reaction. ...
0
votes
0answers
35 views

How to sweep a parameter in an ODEs system using ParametricNDSolveValue?

I want to analyze how my ODEs system behaves when I change only one parameter (in this case d). ...
0
votes
0answers
50 views

Solving a big System of ODEs

I want to solve numerically a linear first-order ODE system with over 12000 equations. I used NDSolve with implicit options: ...